shellDBus: Accept connector to restrict OSD to one monitor
The monitor index is not something that gnome-shell and
gnome-settings-daemon seem to be agreeing about. Using the connector
string is a much more reliable method of identifying a specific screen
and we are indeed using this already for monitor labling.
So switch over to use the connector rather than the monitor index. If a
user tries to use the old API, then the OSD will simply show up on all
monitors (which is the status quo currently anyway).